// Copyright 2016 The Nomulus Authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package google.registry.util;

import static com.google.appengine.api.ThreadManager.currentRequestThreadFactory;
import static com.google.common.base.Preconditions.checkNotNull;
import static java.lang.Math.max;
import static java.lang.Math.min;
import static java.util.concurrent.Executors.newFixedThreadPool;

import com.google.common.base.Function;
import com.google.common.collect.FluentIterable;
import com.google.common.collect.ImmutableList;
import com.google.common.util.concurrent.UncheckedExecutionException;
import com.google.common.util.concurrent.Uninterruptibles;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadFactory;

/** Utilities for multithreaded operations in App Engine requests. */
public final class Concurrent {

  /** Maximum number of threads per pool. The actual GAE per-request limit is 50. */
  private static final int MAX_THREADS = 10;

  /**
   * Runs transform with the default number of threads.
   *
   * @see #transform(Collection, int, Function)
   */
  public static <A, B> ImmutableList<B> transform(Collection<A> items, final Function<A, B> funk) {
    return transform(items, max(1, min(items.size(), MAX_THREADS)), funk);
  }

  /**
   * Processes {@code items} in parallel using {@code funk}, with the specified number of threads.
   *
   * <p><b>Note:</b> Spawned threads will inherit the same namespace.
   *
   * @throws UncheckedExecutionException to wrap the exception thrown by {@code funk}. This will
   *     only contain the exception information for the first exception thrown.
   * @return transformed {@code items} in the same order.
   */
  public static <A, B> ImmutableList<B> transform(
      Collection<A> items,
      int threadCount,
      final Function<A, B> funk) {
    checkNotNull(funk);
    checkNotNull(items);
    ThreadFactory threadFactory = currentRequestThreadFactory();
    if (threadFactory == null) {
      // Fall back to non-concurrent transform if we can't get an App Engine thread factory (most
      // likely caused by hitting this code from a command-line tool). Default Java system threads
      // are not compatible with code that needs to interact with App Engine (such as Objectify),
      // which we often have in funk when calling Concurrent.transform().
      // For more info see: http://stackoverflow.com/questions/15976406
      return FluentIterable.from(items).transform(funk).toList();
    }
    ExecutorService executor = newFixedThreadPool(threadCount, threadFactory);
    try {
      List<Future<B>> futures = new ArrayList<>();
      for (final A item : items) {
        futures.add(executor.submit(new Callable<B>() {
          @Override
          public B call() {
            return funk.apply(item);
          }}));
      }
      ImmutableList.Builder<B> results = new ImmutableList.Builder<>();
      for (Future<B> future : futures) {
        try {
          results.add(Uninterruptibles.getUninterruptibly(future));
        } catch (ExecutionException e) {
          throw new UncheckedExecutionException(e.getCause());
        }
      }
      return results.build();
    } finally {
      executor.shutdownNow();
    }
  }

  private Concurrent() {}
}